fix: backend/pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-ORGMORTBAYJETTY-1021919
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHEMAVENSHARED-570592
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHEHTTPCOMPONENTS-30646
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHECOMMONS-1316641
- https://snyk.io/vuln/SNYK-JAVA-ORGMORTBAYJETTY-6243615
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHECOMMONS-1316638
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHECOMMONS-1316639
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHECOMMONS-1316640
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHECOMMONS-32122
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHECOMMONS-32473
- https://snyk.io/vuln/SNYK-JAVA-ORGCODEHAUSPLEXUS-31680
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHEHTTPCOMPONENTS-1048058
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHEHTTPCOMPONENTS-31517
- https://snyk.io/vuln/SNYK-JAVA-ORGMORTBAYJETTY-173762
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHEHTTPCOMPONENTS-30644
- https://snyk.io/vuln/SNYK-JAVA-ORGAPACHEHTTPCOMPONENTS-451097

backend/pom.xml

@@ -146,7 +146,7 @@
         <dependency>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-site-plugin</artifactId>
-            <version>3.7.1</version>
+            <version>3.12.1</version>
             <type>maven-plugin</type>
         </dependency>